1. Field of the Invention
This invention relates in general to rotary well drilling, and in particular to a shock absorbing apparatus placed in the drill string.
2. Description of the Prior Art
Shock absorbers of various types are used in the drill string of rotary well drilling apparatus. One type uses gas as the shock absorbing medium, as is shown for example in U.S. Pat. Nos. 3,382,936, 3,746,329 and 4,055,338. In each of these patents a fluid filled chamber between a mandrel and a tubular body absorbs vertical shock loading during well drilling operations. To transmit torque while allowing telescoping movement, splines or pins are used between the mandrel and body. Thus, the rotational movement of the drill string is transferred to the drill bit.
In U.S. Pat. No. 3,382,936, splines or ridges extending vertically along the exterior of the mandrel engage grooves on the interior of the body. As shocks are encountered, the splines ride up and down in the grooves while transferring torque from the body to the mandrel. In U.S. Pat. No. 4,055,338, the splines of '936 are replaced by cylindrical pins. These pins are inserted in cavities formed by mating grooves on the interior of the body and exterior of the mandrel. The pins permit sliding or vertical movements of the mandrel relative to the body.
The previous splines or pins used for torque transmission have significant disadvantages. Splines are relatively expensive to machine, particularly if close tolerances are maintained. If loose tolerances are permitted, some splines will be wider than others; some splines will deviate excessively from parallel alignment with the others. As a result, stress and wear will be unequal between the splines and the torque transmission capability will be less than that for which the system was designed.
In the known prior art shock absorbers which use pins as a substitute for milled splines there are also significant disadvantages. The pins are difficult to accurately match with the receiving slots in the mandrel and in the body. This causes some of the pins or splines to experience a greater share of the torque load and stress than others. Sometimes, a single pin or spline must bear the entire load. Correcting this problem by more precise machining is considered prohibitively expensive. Another problem is encountered due to the pin contacting sharp edges along the receiving slots of the body or mandrel. Such edges also create wear problems because of the large resultant contact stresses.